【目的】探究种子吸水萌发过程中水分相态的变化,为种子吸水、萌发研究提供一种新的研究手段。【方法】以初始温度85 ℃热水处理后的刺槐种子为材料,采用质量法确定种子的吸水曲线,低场核磁共振技术(L-NMR)采集刺槐种子吸水、萌发过程中横向弛豫时间(T₂)的信号,并反演得到T₂弛豫谱,分析此过程中种子体内水分相态及含量的变化。【结果】热水处理后刺槐种子的吸水率远远高于对照组,0~12 h为快速吸水阶段,之后吸水速度变缓,至36 h时吸水渐趋于平衡。核磁共振波谱图表明,刺槐种子水分质量(x)与核磁共振弛豫图谱峰面积(y)呈一元线性回归关系,其线性回归方程为:y = 21 132x + 698.05,R²=0.999 6。刺槐种子在吸水萌发过程中存在3种相态的水:束缚水(T₂₁,>0.1~1.0 ms)、不易流动水(T₂₂,>1~100 ms)、自由水(T₂₃,>100~1 000 ms)。吸水萌发过程中束缚水的峰顶点变化不显著,弛豫范围、峰面积总体呈先上升后下降的趋势,但峰比例总体呈下降趋势,吸水24 h后,比例维持在4%以下,胚根伸出时,束缚水消失。在吸水3~9 h过程中,不易流动水的峰显著向右偏移,随后峰顶点时间趋于稳定(9~96 h);胚根伸出时,峰再次显著向右偏移;弛豫时间范围基本呈不断增大的趋势,胚根伸出时又显著减小;其峰面积总体呈先迅速上升(3~12 h)后保持基本稳定的趋势,但比例略有下降。自由水峰顶点随时间呈先上升后下降的趋势,且在吸水72 h时达到最大值;峰面积及比例的最大值出现在胚根伸出时(3 h时峰面积最大值是最小值的4.16倍)。【结论】刺槐种子在吸水萌发过程中存在3种相态的水,其中不易流动水占比最大,各相态水的含量处于一个动态变化过程;随吸水时间的延长,种子内部营养物质开始分解转化,水分结合能力变弱,特别是胚根穿过种皮时,种子代谢活动旺盛,自由水含量大幅增加。

【Objective】 Water is important for seed germination, and nutrients in seeds require water to undergo physiological and biochemical reactions. This study used a nuclear magnetic resonance technology to explore water absorption of Robinia pseudoacacia seeds after hot-water treatments in order to explore changes in water phases during seed imbibition and germination. This study also provides a theoretical basis for water absorption. 【Method】Seeds of R. pseudoacacia were treated with hot water at an initial temperature of 85 ℃. Water absorption was calculated through dividing the increase in weight after soaking by the initial weight. The low nuclear magnetic resonance (L-NMR) was used to collect transverse relaxation time (T₂) signals of R. pseudoacacia seeds during imbibition, and a T₂ relaxation spectrum was obtained by inversion. Changes in water phases and abundances of each phase in the seeds during water absorption were analyzed. 【Result】The water absorption rate of seeds treated with hot water was markedly higher than that of seeds in the control group; the seeds entered the rapid water absorption stage from to 0-12 h, after which their water absorption rate decelerated, and saturation was reached at 36 h. The NMR spectrum showed that the water content (x) of R. pseudoacacia and the peak area (y) had a significant linear relationship, and the linear regression equation was y = 21 132x +698.05; R² = 0.999 6. There were three phases of water in R. pseudoacacia seeds during water-absorbing germination: bound water (T₂₁, >0.1-1 ms), immobile water (T₂₂, >1-100 ms) and free water (T₂₃, >100-1 000 ms). During imbibition, the peak time of bound water did not change significantly, and the relaxation range and peak area generally increased first and then decreased. However, the proportion of the peak area decreased. After 24 h of water absorption, the proportion remained below 4%, and the proportion of bound water in the seeds was very small. Bound water disappeared completely when the radicle passed through the seed coat. During 3-9 h of water absorption, the peak time of immobile water shifted significantly to the right, after which the peak time tended to be stable (approximately 9-96 h). When the radicle passed through the seed coat, peak time again shifted significantly to the right. The relaxation range of immobile water showed an increasing trend. However, it decreased significantly when the radicle passed through the seed coat. The peak area increased rapidly at first (approximately 3-12 h) and then remained stable, but the proportion of peak area decreased slightly, especially when the radicle passed through the seed coat. The peak time of free water changed the most. It reached a maximum at 72 h and then decreased significantly. That is, peak time shifted to the right first and then to the left. The relaxation ranges of free water also first showed an increasing trend and then decreased when the radicle passed through the seed coat. The peak area of free water increased rapidly at first (approximately 3-24 h) and then remained stable. When the radicle passed through the seed coat, it reached a maximum, which was 4.16-fold the minimum value (3 h). The proportion of peak area showed an increasing trend, especially when the radicle passed through the seed coat. 【Conclusion】There are three phases of water in R. pseudoacacia seeds during imbibition, and the most abundant water phase was immobile water. Each phase of water was in a state of dynamic change during imbibition. With the extension of water absorption, nutrients within the seeds began to decompose and transform, decreasing their ability to bind water. In particular, when the radicle passed through the seed coat, the content of free water increased significantly, and the seeds’ metabolism increased markedly.